This invention relates generally to variable reluctance sensors used to detect the angular velocity and/or acceleration of a rotating member. More specifically, the invention relates to such sensors in applications requiring resolution and signal level adequate to accurately detect relatively low angular velocities such as vehicle anti-lock braking and traction control.
The general manner in which a sensor output signal is developed by a variable reluctance sensor is by varying a magnetic flux through a main flux loop surrounded in part by coil windings, thereby inducing a current in the coil. This is typically accomplished by using a toothed ring proximate as possible with minimal air gap to at least one magnet and coil to provide a varying magnetically permeable path for the flux by presenting alternating teeth and spaces to the magnet as the toothed ring rotates. This varies the flux through the coil resulting in a periodic electrical signal at output leads. It is desirable to maximize the flux coupled through the main flux loop since this effects the amplitude of the output signal. This is generally accomplished by minimizing the air gap and providing high permeability flux paths through as much of the main flux loop as practical.
Variable reluctance sensors described in the prior art are frequently depicted proximate to, and at times integral with, vehicle wheel bearing assemblies. A bearing has a rotatably static race coupled to a vehicle chassis and a rotatably dynamic race coupled to a vehicle wheel to rotate at the wheel's angular velocity. Some prior art devices have mounted to, or integral with, a portion of the rotating race a toothed ring made of a suitable magnetically permeable material. Proximate to the outer periphery of such a ring, and rotatably static, is one or more magnets, possibly a magnetically permeable flux member adjacent thereto and coil of wire with a pair of output leads. Arrangements for the magnet and coil vary from generally probe type having a coil wrapped about a flux member to ring type having an annular magnet and coil configuration.
Probe type sensors are mounted either through the bearing housing or external thereto in order that the magnet or flux member is placed in proximity to the toothed ring. The air gap between the toothed ring and the probe is critical to obtaining maximum sensor output but is a burdensome tolerance to control in a manufacturing environment. Annular type sensors used in conjunction with a toothed ring mounted to, or integral with, a portion of the rotating race are disclosed which mount concentrically on the outer peripheral edge of a bearing grease seal radially aligned with the toothed ring which is proximate to the inner surface of the seal. Such an arrangement has a seal separate from the sensor assembly resulting in proliferation of component parts as well as increased gaps between the sensor assembly and the toothed ring approximating the thickness of the seal therebetween. Further, annular magnets utilized in such sensors tend to be costly because of their configuration and amount of magnetic material.
Another annular type sensor shown in the prior art integrates the sensor and a toothed ring or equivalent within a bearing seal. The integral package has inner and outer annular members, a stationary one of which carries at least the coil, with the remaining components being divided between the two members. The inner annular member couples to the inner race and provides a seal therebetween. The outer annular member couples to the outer race and likewise provides a seal therebetween. Some type of dynamic annular seal(s) between the two rotating annular members is also required in addition to the seals at the two races. Subassembly of the sensor is required and tolerances of the package must simultaneously account for proper fit to the inner race and outer race, between the two annular members at the internal seal(s) and between sensor elements.